A flat panel display (FPD), such as a liquid crystal display and so forth, employing a TFT substrate, having thereon an array of switching elements, such as thin-film transistors (TFTs), are being put to practical use.
In the liquid crystal display, TFTs, pixel electrodes, a counter-electrode, and the wiring connected thereto, are arranged on a TFT substrate or a counter-substrate opposite to the TFT substrate, with a liquid crystal sandwiched in-between. Whether the counter-electrodes are provided on the TFT substrate or on the counter-substrate depends on the liquid crystal or the display mode used. The TFT substrate is ordinarily a flat plate, such as a glass substrate having a rectangular-shaped display area on which there is formed an array of TFTs, pixel electrodes, storage capacitors and the counter-electrodes, as necessary. There are provided one or more rectangular-shaped panels each including a display area.
FIGS. 28A to 28C are schematic diagrams illustrating a typical TFT PLD disclosed in Patent Document 1. A large number of TFT panels 50 are formed on a sole glass substrate 40 using a lithographic technique and a semiconductor production process (FIG. 28A). Each TFT panel 50 is formed by an array of a large number of pixel electrodes. A large number of pixels 51 are arranged in a matrix configuration of columns and rows, as shown in FIGS. 28B and 28C.
In FIG. 28C, display on each TFT panel 50 is by addressing a row Lr and a column Lc of each pixel 51 by a selection signal.
FIG. 29 shows an illustrative configuration of a TFT panel for display as disclosed in Patent Document 3. Each TFT panel is made up of a thin-film transistor (TFT) a, a pixel electrode b, an odd data line e, an even data line f, an odd gate line c, an even gate line d and a common line g. The data lines e, f and the gate lines c, d cross one another but are not electrically connected one with another. Each TFT a is electrically connected to the data lines e, f and the gate lines c, d. It is observed that there are TFT arrays not having the common line g. In this case, the pixel electrode b is connected via a static capacitance to a neighboring gate line.
In order for the display to be in operation, each TFT must operate normally. Moreover, the voltage must be applied to the pixel electrodes to display a picture. To check for whether or not the voltage is being normally applied to the pixel electrode, the fact that the kinetic energy of secondary electrons which are generated on irradiating charged particles on the pixel electrodes is changed by the voltage of a pixel electrode, may be utilized.
FIG. 30 shows a configuration disclosed in Patent Document 1. Referring to FIG. 30, there is provided an electron beam source 11 that irradiates an electron beam EB on an array substrate 5 of the display. Secondary electrons SE, generated on irradiation of the electron beam, may be detected by an electron detector DE (secondary electron detector). Emission of the secondary electrons SE is directly proportionate to the voltage of a pixel 6 disposed on the substrate emitting the secondary electrons. An output of the electron detector DE represents the voltage of the pixel, as an object under test, and is delivered to a signal generator/signal analyzer 8. A driving signal, supplied to a terminal of a TFT of each pixel, is formed by the signal generator/signal analyzer 8 and transmitted over lines 61, 62 to the pixel 6. This driving signal is scanned in synchronization with scanning of the electron beam EB indicated as double arrows S (refer to Patent Documents 1 and 2). This technique of voltage contrast of the charged particles, also termed the ‘electron beam test technique’, is a non-contact method for verifying the state of each TFT on the substrate. The technique has an advantage of lower cost in comparison with the conventional test method that uses a mechanical probe, while having an advantage of a faster test speed in comparison with the optical test method.
Based on the disclosure of Patent Document 3, the principle of the voltage technique, which is based on the amount of detection of secondary electrons, is described.
The amount of the secondary electrons, emitted from the pixel electrodes of the TFT substrate to get to the electron detector DE, is dependent on the polarity of the voltage of the pixel electrode. For example, if the pixel electrode is at a positive (plus) potential, the secondary electrons, generated by irradiation of charged particles on the pixel electrodes, are drawn into the pixel electrodes because of the negative (minus) potential of the secondary electrons. As a result, the quantity of the secondary electrons, getting to the electron detector DE, is decreased.
If conversely the pixel electrode has a negative (minus) potential, the generated secondary particles, having charges of the negative potential, and the pixel electrode, repel each other. Hence, the secondary electrons generated get to the electron detector DE without decreasing in their quantity.
The waveform of secondary electrons, corresponding to the voltage waveform of the pixel electrode, may thus be measured by exploiting the fact that the quantity of secondary electrons corresponding to the voltage waveform of the pixel electrode is influenced by the polarity of the voltage of the pixel electrode. The voltage polarity may be positive, negative or zero corresponding to zero-voltage application.
That is, the voltage waveform may be known indirectly, such that, by comparing it to a predicted waveform of the secondary electrons, it may be checked whether or not the voltage is being applied as regularly to the pixel electrode.
The pixel electrode of the TFT array is normally rectangular or polygonal in shape and may be tens to hundreds of micrometers (μm).
The size of the pixel electrode depends on the size and resolution of the display as a completed product. Hence, in testing TFT arrays with differing size and/or resolution, by a sole TFT array test apparatus, it is necessary to test pixel electrodes differing in size.
On the other hand, with a conventional TFT array test apparatus, employing a charged electron beam, a beam of charged particles of a preset diameter is swept on a TFT substrate, and secondary electrons are detected at a preset timing to acquire a waveform of secondary electrons.
FIGS. 31A to 31E are diagrams for illustrating the sweeping of the beam of charged particles and detection of secondary electrons. It is observed that these views show the case where four detection points are obtained for one pixel electrode, and that the secondary electrons are detected at the timings and transverse coordinates of the pixel electrodes indicated by α, β and γ
FIG. 31A shows a position of a first detection point α1-1 on a pixel electrode at a coordinate position (α1).
FIG. 31B shows a position of a second detection point α1-2. The beam of the charged particles then moves to a pixel electrode at a neighboring coordinate position (β1) to detect a first detection point (β1-1) on the pixel electrode (FIG. 31C).
After the end of the scanning of the first row of the TFT substrate, the beam of the charged particles proceeds to sweeping the second row to detect the secondary electrons at detection points on the pixel electrode (FIGS. 31D and 31E).
By the repetition of detection of the signal of the secondary electrons by this sweeping at preset timings, four points on the pixel electrode may be detected (see internally hatched circles in FIG. 3E).
[Patent Document 1] JP Patent Kokai JP-A-2000-3142
[Patent Document 2] U.S. Pat. No. 5,982,190
[Patent Document 3] JP Patent Kokai JP-A-2005-217239